It is generally known to convert toxic components of exhaust gases of an internal combustion engine as far as possible into non-toxic gases. Such toxic components are, for example, HC, NOx and CO which are converted by a catalytic converter mounted in the exhaust-gas system of the engine. It is decisive for the so-called conversion degree that the oxygen content of the exhaust gas lies within optimal values. For a so-called three-way catalytic converter, these optimal values lie within a narrow range about the value which corresponds to a fuel/air mixture of .lambda.=1. In order to maintain this narrow range, it is conventional to control the fuel/air ratio for an internal combustion engine by means of oxygen probes which are mounted in the exhaust-gas system of the engine. In order to accelerate the control operation, especially in transition regions, a determination of a so-called precontrol value based on operating characteristic variables of the engine is made especially on the basis of the air quantity Q supplied to the engine and the engine speed n. This determination of the precontrol value is in addition to the control based on the signal of the oxygen probe. The determination of the air quantity Q can take place in different ways such as via determination of the opening angle of the throttle flap or based on the signal of an air-quantity sensor.
The precontrol value, which is determined on the basis of Q and n, is corrected in accordance with the signal of the oxygen probe in such a manner that the optimal fuel/air mixture is determined. With the corrected signal, a fuel-metering device is driven which supplies the optimal quantity of fuel to the engine. This can take place, for example, by controlling the injection valves with an injection pulsewidth ti.
Furthermore, systems are known which utilize two lambda probes as oxygen probes to control the fuel/air mixture. A first one of these probes is arranged forward of the catalytic converter and a second probe is connected downstream thereof. The signal of the second lambda probe is compared to a desired value and the difference of these two values is integrated and the value obtained in this manner operates on the desired value of the control based on the signal of the lambda probe. Such a system is disclosed, for example, in U.S. Pat. No. 5,224,345. It has also been shown that present day three-way catalytic converters exhibit a capability to store gas especially a capability to store oxygen. In this context, U.S. Pat. No. 5,207,056 discloses that the gas storage capacity of a catalytic converter is to be considered for the control. In accordance with this teaching, not only the instantaneous value of .lambda. forward of the catalytic converter is controlled to a desired value but the integral of the control deviation is also controlled to a desired value via a superposed control with the aid of the probe rearward of the catalytic converter.